The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to an improved system for maneuvering streamers and acoustic energy sources through water in marine seismic operations.
Marine seismic exploration investigates the structure and character of subsurface geologic formations underlying a body of water. For large survey areas, seismic vessels tow one or more seismic sources and multiple seismic streamer cables through the water. The seismic sources typically comprise compressed air guns for generating an acoustic pulse in the water. The energy from such pulse propagates downwardly into the geologic formations and is reflected upwardly from the interfaces between subsurface geological formations. The reflected energy is sensed with hydrophones attached to the seismic streamers, and data representing such energy is recorded and processed.
Three dimensional ("3-D") seismic surveys provide more information regarding the subsurface formations than two dimensional seismic surveys. 3-D surveys may be conducted with up to twelve or more streamers which form an array covering a large area behind the vessel. The streamers typically vary in length between three and twelve kilometers. Tail buoys attached at the streamer distal ends carry radar reflectors, navigation equipment, and acoustic transponders. Hydrophones are positioned along each streamer and are wired together in receiver groups spaced along each streamer. The in-line interval between receiver groups ranges between 5 and 25 meters, with 12.5 meters comprising a typical interval spacing.
A multiple streamer array requires diverters near the vessel to pull the streamers outwardly from the direct path behind the seismic vessel and to maintain the cross-line spacing between individual streamers. Diverters rely on hydrodynamic lift created by forward motion through the water to pull the streamers outwardly and to maintain the transverse position relative to the vessel path. If forward motion changes due to ocean currents and other environmental factors, the diverters will not maintain the optimum streamer orientation.
In-line spacing between receiver groups, and cross-line spacing between streamers, is critical to the efficient collection and analysis of geophysical data. Consistency in the orientation of seismic assets affects imaging of the subsurface. For example, surface sampling of receiver groups affects accurate detection of the reflected seismic signals and the "noise" interfering with such signals. Closer receiver group or streamer spacing will increase the accuracy of acoustic wavefield represented by the recorded data, while wider spacing between streamers permits a larger area to be surveyed for each seismic vessel pass.
The deployment, operation, and retrieval of multiple streamers requires handling and time. Each day on prospect is expensive and significantly increases survey costs. The streamers are carried by the seismic vessel and are deployed into the water after the survey site has been reached. At the end of each survey line, the vessel turns around and charts the next pass. Vessel turns are complicated by the long streamers extending behind the vessel, and the turning radius is typically large to minimize the possibility of streamer entanglement. When the survey is complete, the streamers are reeled onto the vessel deck for relocation to the next survey site.
Multiple vessel configurations can collect certain information not available from a single seismic vessel, such as when an obstruction blocks passage of a single seismic vessel. One technique involves "undershooting" of obstructions. A primary vessel and receiver spread sails on one side of an obstruction, and a secondary vessel towing the energy source sails on the other side of the obstruction. Subsurface coverage underneath the obstruction is obtained between the two offset vessels. Because both vessels are sailing in the same direction, subsurface coverage is offset in the transverse or cross-line direction relative to the receiver spread, and fold is acquired in the in-line direction only. Another multiple vessel technique uses a secondary vessel to acquire offset information exceeding the streamer length. The secondary vessel is offset from the receiver spread in the in-line direction ahead of the primary vessel or behind the tail end of the receiver spread. Both vessels sail in the same direction, resulting in in-line fold build-up.
Another geophysical exploration technique known as "zig-zag shooting" is a hybrid of conventional streamer methods combined with zig-zag shooting geometry. A "master" vessel tows several streamers and seismic sources in a conventional parallel path, and a second "slave" vessel provides a secondary source while sailing a continuous 45 degree reversing angle route. The sources generate acoustic energy in alternating modes between the two vessels. Because conventional streamer data having energy sources and streamers with a single vessel is collected simultaneously, the cross-line separation cannot be altered during data acquisition. The resulting seismic data attributes represent a mixture of back azimuth zig-zag (patch) shooting and forward azimuth conventional in-line (swath) shooting.
In addition to the deployment and operation difficulties associated with towing multiple streamers, conventional techniques limit the ability to position source equipment and receivers in different relative positions and orientations. Because the sources and receivers are towed behind the same seismic vessel, array design is inherently limited by the tow configuration. Each towed array is also subject to cross-currents, wind, waves, shallow water, and navigation obstacles which limit the coverage provided by the survey system. Conventional tow systems experience significant drag and lead-in sag which require additional tow force to overcome. The large forces experienced by constituent components of conventional tow configurations sometimes exceed the mechanical limits of the components.
Accordingly, a need exists for an improved technique for conducting marine seismic operations. The technique should be economic, flexible and extensible. Additionally, the technique should facilitate repair without disrupting geophysical operations and permit various streamer and source geometric configurations to be implemented.